1. Technical Field of the Invention
The invention relates to the field of radio frequency (R.F.) circuits and in particular to multiband R.F. circuits adapted to two or more R.F. frequency bands like the frequency bands defined for the global system for mobile communication (GSM), e.g. 450 MHz (GSM 450), 900 MHz (GSM 900), 1800 MHz (GSM 1800) and 1900 MHz (GSM 1900).
2. Description of the Prior Art
R.F. circuits are utilized for a large variety of different applications. As an example, antenna switching circuits for mobile telephones can be mentioned. Mobile telephones adapted to the time division multiple access (TDMA) mode, for example GSM systems, are commonly using antenna switches for coupling an antenna port to either a transmitter path or a receiver path of the mobile telephone.
An antenna switch for a single frequency band and consisting essentially of two pin-diodes and a quarter-wavelength transformer is known from WO 88/00760. The antenna switch is depicted in FIG. 1. In a transmit mode, both pin-diodes D1 and D2 are switched on. A transmitter port PTX is thus connected to an antenna port PANT via a first pin-diode D1. A receiver port PRX is connected to ground via a second pin-diode D2 and the resulting short circuit at receiver port PRX is transformed by the quarter-wavelength transformer to an open circuit at antenna port PANT. Receiver port PRX is thus isolated from antenna port PANT. In a receive mode both pin-diodes D1 and D2 are switched off. In the receive mode, transmitter port PTX is virtually disconnected from antenna port PANT and receiver port PRX is connected to antenna port PANT via the quarter-wavelength transformer. The switching state (on/off) of pin-diodes D1 and D2 is controlled by means of a control voltage VDC applied to a control port. Inductor L1 provides a DC path to pin-diodes D1 and D2 and resister R1 sets the DC current through pin-diodes D1 and D2.
For mobile telephones operable in a dual frequency band mode or in a triple frequency band mode the antenna switch depicted in FIG. 1 has to be modified. In dual band applications like GSM 900/GSM 1800 or GSM 900/GSM 1900 for example a diplexer circuit may be inserted into the common antenna path. The diplexer circuit splits incoming antenna signals into high-band signals and low-band signals. The incoming high-band signals and low-band signals are thereafter individually applied to separate antenna switches. Thus a first antenna switch for high-band signals and a second antenna switch for low-band signals has to be provided, each antenna switch further splitting the antenna path into a transmitter path and a receiver path. Triple-band applications like GSM 900/GSM 1800/GSM 1900 usually also utilize a single diplexer circuit for splitting the common antenna path into a low-band signal path (GSM 900)/and a combined high-band signal path (GSM 1800/GSM 1900).
The use of a diplexer circuit for splitting a signal incident at the antenna port into low-band and high-band signals leads to a rather complex circuit design. Therefore, antenna switches configured to also perform the signal splitting function of a diplexer circuit have been proposed.
An antenna switch for coupling a single antenna to either one of a first and a second receiver, operable at a first and a second frequency band, respectively, and a first and a second transmitter, operable to transmit at the first and the second frequency band, respectively, is known from DE 197 04 151. The antenna switch has a multiband transformation stage 100 as schematically depicted in FIG. 2. The multiband transformation stage 100 comprises a common signal input 102, two separate signal outputs 104, 106, two quarter-wavelength transformers SL1, SL2 coupled in series and three switching elements SE3, SE4, SE5.
The two quarter-wavelength transformers SL1, SL2 coupled in series represent together a quarter-wavelength transmission line at a first frequency band and each single quarter-wavelength transformer SL1, SL2 represents a quarter-wavelength transmission line for a second frequency band equaling approximately twice the first frequency band.
Signal input 102 of the multiband transformation stage 100 is usually coupled to an antenna and to a multiband transmitter switch for coupling either a first transmitter operable in the first frequency band or a second transmitter operable in the second frequency band to the antenna. First signal output 104 may be coupled to a first receiver receiving in the first frequency band and second signal output 106 may be coupled to a second receiver receiving in the second frequency band.
The multiband transformation stage 100 has four operational states. In a first operational state corresponding to transmission in the first frequency band, switching elements SE3 and SE4 are switched off and switching element SE5 is switched on. The short circuit created by switching element SE5 at a node 108 is transformed to an open circuit for the first frequency band at signal input 102 of the multiband transformation stage 100. In a second operational mode corresponding to transmission in the second frequency band, switching element SE3 is switched on and switching elements SE4 and SE5 are switched off. Switching element SE3 thus creates a short circuit at a node 110. This short circuit is transformed by the quarter-wavelength transmission line SL1 to an open circuit for the second frequency band at signal input 102. In a third operational state corresponding to receiving in the first frequency band, switching elements SE3, SE4 and SE5 are turned off. Consequently, first signal output 104 is coupled impedance-matched via the two quarter-wavelength transmission lines SL1, SL2 with signal input 102. In a fourth operational state corresponding to receiving in the second frequency band, switching element SE3 is turned off and switching elements SE4, SE5 are turned on. This means that second signal output 106 is coupled impedance-matched via first quarter-wavelength transmission line SL1 with signal input 102. Further, the short circuit created by switching element SE5 is transformed by second quarter-wavelength transmission line SL2, which has a quarter-wavelength characteristic for the second frequency band, into an open circuit at second output port 106.
The fourth operational stage necessitates that the two quarter-wavelength transmission lines SL1, SL2 have an identical transformation characteristic. This requirement, however, limits the applicability of the multiband transformation stage 100 depicted in FIG. 2 to the case where the first frequency band equals approximately half the second frequency band. A further disadvantage of the multiband transformation stage 100 is the fact that in the fourth operational state, i.e. in high-band receive mode, two switching elements SE4, SE5 are in an on state. This leads to a considerable current consumption in the order of milliamperes and reduces the stand-by time of battery-powered devices. Moreover, the multiband transformation stage 100 comprises altogether three switching elements SE3, SE4, SE5 which have to be biased. This requires a comparatively complex biasing network. The biasing network becomes even more complex if the multiband transformation stage 100 has to be adapted for triple-band applications.
Also, the multiband transformation stage 100 suffers from limited isolation between signal input 102 which may be coupled to transmitters and signal outputs 104, 106 which may be coupled to receivers. This means that terminations of signal outputs 104, 106 become relevant in the first two operational states, i.e. in transmit modes. Terminations of output ports 104, 106, however, are difficult to design due to the constraints imposed by the receivers coupled to output ports 104, 106.
A further multiband switching device with a multiband transformation stage is known from WO00/41326. The multiband switching device is utilized for switching an antenna port between two power amplifier ports and a receive port. The multiband transformation stage comprises one or more diode switches positioned between a first one of the amplifier ports, a second one of the amplifier ports and ground. One or a plurality of frequency dependent isolation sections are positioned between the first amplifier port, the second amplifier port and the receive port so that the diode switches are controlled using a respective control input for connecting either the first amplifier port, the second amplifier port or the receive port to the antenna port. As far as the low-power side of the multiband switching device known from WO00/41326 is concerned, the transformation stage suffers from a low flexibility.
There is, therefore, a need for a multiband switching device which does not suffer from the limitations of the prior art switching devices.